This invention relates to Optical Scanners, and more particularly to a circuit for detecting differential changes in the output of a photocell or other optical transducer.
Photodetection of indicia on forms and the like is known. In such systems a narrow beam of light is focused as a spot or slit of light on the relevant field to be scanned and is moved over the field, either by the deflection of the beam or movement of the form. Light reflected from, or transmitted through, the form is focused on a photocell or other photodetector so that indicia may be detected as variations in the photocell output. Typically, photocell outputs are small so that the photocell is connected to an amplifier and the amplified output is further processed by the system.
One application of photodetection is in the control of Computer Output Mailing Systems (COMS). Such systems receive fan-fold computer printout, separate the printout into discrete documents, assemble the documents and other materials into batches to be mailed together, and insert the batches into envelopes. Such COMS are typically controlled by information encoded on "control documents" as "dash codes". "Dash codes" consist of lines, normal to the direction of motion of the printout, which are printed on the control document, typically on the sprocket strips of the printout. The field containing the "dash codes" is scanned by reflected light as the document is received by the COMS and the presence or absence of lines encodes information defining the operations relating to that control document.
Scanning such computer printout is complicated by the fact that the color or shade of the printout may vary from job to job, changing the background or reference level against which the "dash codes" must be detected. Also, the intensity of the light source used may vary with time shifting the signal levels produced by the photodetector.
Heretofore compensating for these problems has required complex calibration circuits and/or frequent adjustments by an operator. Typical of the prior art circuits is that disclosed in U.S. Pat. No. 4,064,833 GLUCK et al issued Dec. 27, 1977; wherein a circuit comprising a second photodiode for monitoring the lamp intensity is disclosed.
Thus, it is an object of the subject invention to provide an optical scanner system and an amplifier for use in such system which are suitable for the detection of objects having a wide spectrum of colors against a wide spectrum of background colors and types. It is another object of the subject invention to provide such a system and amplifier which compensate for changes in illumination intensity.
It is still another object of the subject invention to provide such a system and amplifier which achieve the above objects without the need for operator intervention.
It is still another object of the subject invention to provide such an amplifier wherein the amplifier sensitivity may be easily adjusted.
It is still another object of the subject invention to provide such an amplifier which may incorporate a degree of hysteresis so that the amplifier output having shifted to indicate the detection of an object that output will tend to retain its present state, despite small changes in the amplifier input, until the input clearly returns to the background level. Such hysteresis is desirable for amplifiers used in systems intended to detect threshold levels in relatively slowly changing signals (e.g., proximity detection of approaching objects) but is not desirable in systems intended to detect rapidly changing low level signals (e.g., low contrast dash codes.)